In a typical alarm system within a building, such as a fire or burglar alarm system, many types of sensors, detectors, lights, strobes, sounders and other associated devices may be located throughout the building as part of the system. Groups of these devices are often wired together along one or more pairs of electrical lines used to supply power and communications to the devices. A group of such devices wired on commonly shared pair of lines is often referred to as a line of devices. Many separate lines of devices typically connect back to a control panel that controls the overall operation of the alarm system. A line of devices is usually associated with a certain zone of the building and/or a certain type of device. For example, one floor of a multi-story building may have all of its smoke detectors wired together on a line that connects back to the control panel.
There may be multiple functions associated with each device on a line. For instance, a device may have a horn to sound an alarm upon the detection of smoke and may also contain a strobe light that turns on to guide people to safety during an alarm condition. Industry standards have been developed to govern how the various functions within devices should operate with the control panel and in relation to each other.
Different jurisdictions have different laws that govern the design of alarm systems. For example, some jurisdictions require that horns and strobes be controlled by the panel through separate loops in order that, once fire fighters have arrived, the horns can be silenced while the strobes continue. In those jurisdictions, where each notification appliance includes both a horn and a strobe, the horn and strobe circuits are isolated and separately powered through connections to separate loops. On the other hand, in jurisdictions where the separate loops are not required, it may be desirable to control both the horn and strobe from a single loop to reduce system costs. In order to provide horn/strobe notification appliances which are compatible with both jurisdictions, jumpers may be included between the horn and strobe circuits. When powered by a single loop, the jumpers are left in place. When powered by separate loops, the jumpers are manually removed during installation to isolate the horn and strobe devices. Unfortunately, one or more sets of jumpers may be inadvertently left connected in a two-loop system. The result is a short circuit between the loop which can cause the devices in the loop to operate erratically or even damage the devices during an alarm condition.
Another fault which can occur during installation and even subsequent to installation is a ground fault. Alarm systems are generally not connected to earth ground. Thus, in a 24 volt system, system ground might float at about 12 volts below earth ground as the positive 24 volt level floats about 12 volts above earth ground. Conventional systems include ground fault detectors which identify when there is a short in the system to earth ground. Again, such a short can cause the system to operate erratically.
Once a ground fault is detected in the system, it must be located in order to correct it. Typically, a technician must remove power from the system and use an ohmmeter to find a ground fault.
In accordance with the present invention, alarm system faults can be detected by selectively isolating loops in the system under programmed processor control while monitoring for faults. For example, where all loops but one are isolated from a power supply, the supervisory current through the non-isolated loop will increase where jumpers inadvertently connect that loop to an isolated loop. That increase in current can be compared through a current sensor. Further, where the system ground fault detector indicates a ground fault, the ground fault can be located by selectively isolating individual loops.
An alarm control panel for implementing the present invention may include the usual connectors to plural appliance loops and a voltage control which applies a first voltage to each loop during an alarm state and a reverse voltage to each loop during a supervisory state. In accordance with the invention, the control panel further includes an isolation control which selectively removes the reverse voltage from the selected loops to test for loop circuit faults. The loops to be isolated are selected under software control and faults are indicated to the operator on a control panel display.